The worst that has ever happened to me when I tried an early check-in was that they stored my bags for an hour and then let me have the room when it was cleaned.
Hotels normally have a noon check-out and the 3 PM check-in is to give time to the cleaning staff. The busier the hotel, the harder to get a room a littl early.
I've checked in as early as 10 AM without an issue.
Michael